In general, raw and intermediate materials containing base metals can be divided into two main categories. One category represents a wide variety of raw and intermediate materials which contain non-ferrous base metals and substantial quantities of iron. The principal objective of reduction smelting of such materials is to separate non-ferrous base metals into a highly concentrated liquid phase containing these metals in the metallic and/or sulfidic form, and reject most of the iron in the form of a discard slag containing as low concentrations of non-ferrous metals as expedient economically.
This kind of metallurgical technology is currently applied in processing nickel laterite ores, partially roasted sulfide nickel concentrates, dead roasted copper concentrates (calcines), and various polymetallic raw or intermediate materials including dusts. With the exception of very few obsolete installations, this technology is at present carried out mainly in electric furnaces under reducing conditions.
Even though reduction electrosmelting proved to be quite effective in terms of separating non-ferrous metals from iron, this process involves large capital expenditures and operational costs. Moreover, in most cases it requires a very careful and expensive preparation of the feed. For example, nickel laterite ores must be well prereduced and preheated; nickel sulfide concentrates must be partially and precisely desulfurized as well as agglomerated; copper calcines must contain as low sulfur contents as possible, and so on. Furthermore, a non-ferrous metals producer who opts to use reduction electrosmelting process faces a choice of purchasing electric power or building its own power station. In the first case, the producer becomes economically dependent on the supplier of power, while in the second case even greater capital expenditures are involved. One more shortcoming of reduction electrosmelting technology stems from the fact that the best heat performance of electric furnaces is achieved when the feed is a dense, low porosity material, for example, in the form of briquettes or pellets, whereas materials to be fed into electro furnaces are most often finely divided. When a finely divided material is reduction smelted in electric furnaces, a number of undesirable phenomena occur including poor heat transfer to the feed, unnecessarily high degree of slag superheat and heat flux to the furnace refractory walls, premature and useless burning of solid reductant added to the feed, evolution of sulfur-containing gases, etc. Finally, electric smelting, as opposed to, for instance, flash smelting, has limited capability to utilize the energy from oxidation of sulfidic sulfur contained in concentrates of non-ferrous metals, and, therefore, is not an energy efficient process. In addition reduction electrosmelting is an inflexible process which is very sensitive to variations of feed and slag composition, especially with respect to iron and silica.
The other category of materials containing non-ferrous base metals represents a wide variety of intermediate products which do not contain substantial quantities of iron, if any. The principal objective of reduction smelting in this case consists in producing a melt which is practically oxygen free and is suitable either as a final product or as a semiproduct for further processing, for instance, for refining or alloying with other metals, whatever the case may be. Among numerous materials of this category, the following can be cited: nickel oxide, cobalt oxide, copper oxide, copper sulfide or nickel sulfide or a mixture of NiCu metallics with these sulfides (e.g., concentrates from separation of slowly cooled and then ground nickel-copper converter matte), various hydrometallurgical precipitates including partially oxidized copper cement, hydroxides and/or carbonates of nickel and cobalt, and many others. These materials in most cases are also quite finely divided. Although they can be processed in a number of ways, most often they too are reduced and smelted in electric furnaces, unless an old technology is used, i.e., reverberatory smelting or smelting in converters, for example. A roasting operation to eliminate sulfur is sometimes required prior to reduction smelting.
Thus, it is evident that reduction smelting of finely divided materials has a wide application in metallurgy of non-ferrous base metals. It is, in most cases, carried out in electric furnaces, and this process is characterized by a number of serious shortcomings already mentioned above.
The major incentives in developing new processes for reduction smelting of materials containing non-ferrous base metals are concerned with reducing capital expenditure and operational cost as well as with providing for operational flexibility of processing a wide variety of materials without having to undertake any major engineering and operational changes. A new process should be an energy efficient and pollution free one. As well, it should be able to take full advantage of the fact that most of the materials to be dealt with are already finely divided or, when necessary, can easily be ground.